Various embodiments of current mirrors are known from the book entitled "Halbleiter-Schaltungstechnik" [Semiconductor Circuitry] by Tietze and Schenk, 8th Edition 1986, pp. 62-64 and 94-97. Current mirrors are asked to meet three demands, yet meeting all three at once is difficult to achieve. The first and second are high accuracy and low expenditure for circuitry. In addition, the voltage drop in the input branch and output branch of the current mirror should be as slight as possible. However, precise current mirrors with a slight voltage drop can be achieved only at considerable expenditure for circuitry. Conversely, less expensive current mirrors are either relatively inaccurate or produce a high voltage drop.
An MOS-integrated constant current source is also known from German Patent DE 28 40 740 C2, corresponding to Published UK Application GB 2 034 939 A. FIG. 2 of that patent shows a circuit made up of MOS transistors, in which the dependency of the output current on the output voltage is reduced.